1. Field of the Invention
The present invention relates to an apparatus for estimating the grip factor of wheels and particularly to an apparatus for estimating the grip factor indicating the grip level of wheels in the front of a vehicle in a lateral direction. The invention also relates to a vehicle motion control apparatus equipped with the grip factor estimating apparatus.
2. Description of the Related Art
There is known an apparatus for detecting and judging the state variable of a vehicle and controlling braking force of wheels independently to retain the stability of the vehicle. For example, this type apparatus has been disclosed in JP-A-6-099800. According to the document JP-A-6-099800, a target value of yaw rate is formed on the basis of vehicle velocity and steering angle, so that a judgment is made on the basis of a time derived function of deviation from the actual value of yaw rate as to whether the vehicle is in a state having a tendency to over-steering or a tendency to under-steering. In the case of over-steering, braking slip of front wheels in the outside of turning is increased, that is, braking force of front wheels in the outside of turning is increased. In the case of under-steering, braking slip of rear wheels in the inside of turning is increased. On the other hand, an apparatus for setting a target value of front wheel velocity difference, lateral acceleration or yaw rate on the basis of steering angle and vehicle velocity and controlling a brake and/or engine output has been disclosed in JP-A-62-146754.
Moreover, a variable rudder angle ratio steering apparatus for vehicle to prevent excessive steering of a steering wheel has been disclosed in JP-A-11-099956. An index of lateral force activity ratio or lateral G activity ratio is used in the apparatus. That is, according to the apparatus described in the document JP-A-11-099956, the friction coefficient μ of a road surface is first estimated so that the lateral force activity ratio is calculated. Because cornering power Cp of tires decreases as the friction coefficient μ of the road surface decreases, rack shaft counterforce received from the road surface in a certain rudder angle decreases in accordance with the friction coefficient μ of the road surface. Accordingly, the document JP-A-11-099956 has described that, when the front wheel rudder angle and the rack shaft counterforce are measured actually, the friction coefficient μ of the road surface can be estimated on the basis of comparison between the rack shaft counterforce actually measured relative to the front wheel rudder angle and reference rack shaft counterforce set as an inner model in advance. Moreover, an equivalent friction circle is set on the basis of the friction coefficient μ of the road surface and part of friction force used by longitudinal force is subtracted from the friction force to calculate maximum generated lateral force to thereby define the ratio of currently generated lateral force to the maximum generated lateral force as the lateral force activity ratio. Or the document JP-A-11-099956 has described that a lateral G sensor is provided so that the lateral G activity ratio can be calculated on the basis of lateral G detected by the lateral G sensor.
Because friction between the road surface and each tire is limited, it is necessary not only to control the yawing motion of the vehicle, that is, the posture of the vehicle on a surface on which the vehicle is running, but also to reduce the velocity of the vehicle so that the turning radius intended by the driver can be retained when the vehicle reaches a friction limit and gets into an excessive under-steering state. In the apparatus described in a document “AUTOMOTIVE ENGINEERING HANDBOOK, First Volume, for BASIC & THEORY, issued on Feb. 1, 1990 by Society of Automotive Engineers of Japan, Inc., c. f. pp 179-180” which will hereinafter referred to as “AUTOMOTIVE ENGINEERING HANDBOOK”, the behavior of the vehicle is however judged after each tire reaches a friction limit. For this reason, there is apprehension that cornering force will be reduced to promote under-steering when the velocity of the vehicle is reduced in the condition that each tire reaches a friction limit. Moreover, in the actual control system, there is a dead zone incapable of being controlled. For this reason, the aforementioned control is executed after the behavior of the vehicle occurs to a certain degree.
Moreover, as the curve shape of the road is formed into a clothoid curve, when the driver intends to trace the curve of the road, the steering wheel will be rotated with a gradual increasing amount. Accordingly, when the velocity of approach to the curve is high, side force generated in the wheels does not balance with centrifugal force so that the vehicle shows a tendency to swelling on the outside of the curve. In such a case, the apparatus described in the document “AUTOMOTIVE ENGINEERING HANDBOOK” or in JP-A-62-146754 will operate to control the motion of the vehicle. However, as the control starts at the cornering limit, there is a possibility that the vehicle velocity cannot be reduced sufficiently by the control. There is a possibility that swelling on the outside of the curve cannot be prevented by only the aforementioned control.
Incidentally, in the document “AUTOMOTIVE ENGINEERING HANDBOOK”, the state in which each tire rolls while sideslipping with a slip angle α has been explained as shown in FIG. 2.
That is, in FIG. 2, the tread surface of the tire represented by the broken line touches the road surface at a front end of a contacting surface including a point A and moves to a point B in the direction of movement of the tire while adhering to the road surface. The tread surface of tire begins to slip at the point of time when flow stress based on lateral shear deformation becomes equal to friction force. The tread surface of the tire is departed from the road surface at a rear end including a point C and restored to its original state. On this occasion, force Fy (side force) generated in the contacting surface as a whole is given as the product of the lateral deformed area (hatched in FIG. 2) of the tread portion and the lateral elastic constant of the tread portion per unit area. As shown in FIG. 2, a point of application of side force Fy is located in the rear (left in FIG. 2) far by en (pneumatic trail) from a point O just under the center line of the tire. Accordingly, moment Fy·en in this case is self-aligning torque (Tsa) which acts to reduce the lateral slip angle α.
Next, the case where tires are attached to the vehicle will be described with reference to FIG. 3 which is a graph obtained by simplifying FIG. 2. In steering the wheels of the vehicle, a caster angle is generally formed so that a caster trail ec is provided in order to make it easy to return the steering wheel. Accordingly, the landing point of each wheel is a point O′, so that the moment to restore the steering wheel is Fy·(en+ec).
When the lateral grip state of the tire is reduced to enlarge the slip region, the tread portion is laterally deformed from a figure ABC to a figure ADC in FIG. 3. As a result, the point of application of side force Fy moves forward (from a point H to a point J in FIG. 3) in the direction of the movement of the vehicle. That is, the pneumatic trail en is reduced. Accordingly, even if the same side force Fy is applied, the pneumatic trail en is increased and the self-aligning torque Tsa is increased when the adhesive region is large and the slip region is small (that is, when the lateral grip of the tire is high). On the other hand, the pneumatic trail en is reduced and the self-aligning torque Tsa is reduced when the lateral grip of the tire is lost and the slip region is large.
As described above, when attention is paid to change in pneumatic trail en, the lateral grip factor of the tire can be detected. Because change in pneumatic trail en is expressed in self-aligning torque Tsa, the grip factor indicating the grip level of lateral grip of wheels in the front of the vehicle (hereinafter referred to as grip factor) can be estimated on the basis of the self-aligning torque Tsa. Alternatively, the grip factor can be also estimated on the basis of the allowance of side force relative to the road surface friction as will be described later.
The grip factor is different from the lateral force activity ratio or lateral G activity ratio disclosed in JP-A-11-099956 as follows. In the apparatus described in JP-A-11-099956, the maximum lateral force which can be generated in the road surface is calculated on the basis of the friction coefficient μ of the road surface. The friction coefficient μ of the road surface is estimated on the basis of dependency of cornering power Cp (defined as a value of side force at a slip angle of 1 deg.) on the friction coefficient μ of the road surface. The cornering power Cp is however affected not only by the friction coefficient μ of the road surface but also by the shape of the contacting surface of the tire (the length and width of the contacting surface), the elasticity of tread rubber, and so on. When, for example, water is interposed in the tread surface or the elasticity of tread rubber changes according to the abrasion of the tire and the temperature, the cornering power Cp varies even in the case where the friction coefficient μ of the road surface is kept constant. As described above, in the technique described in JP-A-11-099956, there is no consideration about tire characteristic of each wheel.